Conventional optical fibers generally have a structure, as shown in FIG. 1, in which a soft primary coating layer 2 is overlaid on a glass optical fiber 1 consisting of silica glass having a core and a cladding which is provided outside the core, and a hard secondary coating layer 3 is further overlaid on the primary coating layer 2.
Typically, a UV curable resin having Young's modulus of 3 MPa or less has been used as the primary coating layer 2, and a UV curable resin having Young's modulus of 500 MPa or more has been used as the secondary coating layer 3.
When such an optical fiber ribbon is immersed in water, delamination may occur at an interface between the glass optical fiber 1 and the primary coating layer 2
If case such delamination is caused, uneven force acts on the glass optical fiber induced by the delamination to cause microbending, whereby the transmission loss is increased. In order to prevent the occurrence of the delamination, extraordinary efforts have been made by thoroughly examining the material of each of the coating layers so as to ensure sufficient adhesion between the primary coating layer 2 and the glass optical fiber 1.
However, there is a limitation in ensuring such sufficient adhesiveness while maintaining the function of each of the coating layers. It has therefore been a practice instead to optimize the cable structure and the matrix, or the quality of material for a colored layer, or to adjust manufacturing conditions.
Japanese Patent Application Laid-Open No. 2002-122761 (Patent Document 1), for example, deals with such approaches and discloses a method for maintaining transmission characteristics in good conditions by defining the water absorption coefficient of the coating resin layers of an optical fiber.
Also, Japanese Patent Application Laid-Open No. 2002-372655 (Patent Document 2) discloses a method for reducing an amount of water that reaches the optical fiber by reducing the water absorption coefficient of the colored layer.
The method described in Patent Document 1 retains the water absorption coefficient of the optical fiber at a low level by controlling the storage conditions of the optical fiber, suppresses foaming caused by water contained in the coating, and prevents deterioration in the hydraulic characteristics and low-temperature characteristics. However, this method cannot be an effective measure for preventing the increase in the loss due to the occurrence of microbending which is caused at the time when water reaches the optical fiber under the usage environment mentioned above.
Even the method described in Patent Document 2 cannot completely shut out water that reaches the optical fiber, and thus the loss due to microbending unavoidably increases depending on the usage environment.
Thus, under the present circumstances, there is no extremely effective measure for suppressing the loss increase when an optical fiber is immersed in water.
In light of the problem provided above, the present invention has an object of providing an optical fiber which is unlikely to cause interlayer delamination between a glass optical fiber and a primary coating layer even when the optical fiber is immersed in water.